Ceramic sheathed-element glow plug with electrically conductive powder pellet contacting element and method for making

ABSTRACT

A ceramic sheathed-element glow plug includes a ceramic glow element made of an electrically conductive layer and an electrically insulating layer, in which the conductive layer is made of supply layers and a heating layer. The higher specific electrical resistance of the heating layer allows the temperature of the heating layer and of the combustion chamber to be determined, and the electrical contact between a connecting element and the glow element is established by a contacting element that is composed of a pellet made of an electrically conductive powder.

FIELD OF THE INVENTION

The present invention relates to a ceramic sheathed-element glow plugfor diesel engines.

BACKGROUND INFORMATION

Glow plugs having external ceramic heaters are discussed, for example,in German Published Patent Application No. 40 28 859. Furthermore,German Published Patent Application No. 29 37 884 discusses metallicsheathed-element glow plugs in which the metallic glow filament iswelded to a thermoelement. During the operation of the sheathed-elementglow plug, the temperature of the respective cylinder can be measuredhere by measuring the heat-generated voltage. However, it is believedthat no metallic glow filament is present in a sheathed-element glowplug having a ceramic heating element.

Furthermore, German Published Patent Application No. 198 44 347discusses a sheathed-element glow plug having a connecting element whichis electrically connected to the glow element via a contacting element.The contacting element is shown as a spring in FIG. 1.

SUMMARY OF THE INVENTION

The exemplary ceramic sheathed-element glow plug according to thepresent invention is believed that the advantage that the temperature ofthe glow element is measurable. In a sheathed-element glow plug, thetemperature of the glow element can be measured directly in a selectedarea on the outside of the glow element without additional hardware. Thetemperature is measured in a selected volume that is small in comparisonwith the volume of the entire glow element, so that the error occurringdue to temperature distribution over a large volume can be reduced indetermining the temperature. Also, in the exemplary sheathed-elementglow plug according to the present invention, the heating power can beconcentrated to a selected area of the glow element without modifyingthe cross-section of the conductive layer, so that the surface arearemains constant in the area in which the heating power is to beconcentrated and thus also the interaction area is kept constant. It isalso believed that such a ceramic sheathed-element glow plug can bemanufactured in a cost-effective manner.

In particular, by selecting the appropriate ceramic materials used forthe different areas of the sheathed-element glow plug, it is believedthat it is ensured that the mechanical stability of the heater is notimpaired. By having the measured temperature values processed by acontroller, the temperature can be regulated in the selected area of theglow element. Also, the exemplary sheathed-element glow plug accordingto the present invention may be used in a passive mode as a temperaturesensor after it has performed its heating function. Thus it can bedetermined whether the combustion process is taking place correctly inthe respective cylinder. It is believed that the parameters relevant tothe combustion can be influenced on the basis of this information.

The exemplary ceramic sheathed-element glow plug according to thepresent invention is believed to have the advantage that, due to thegreater conductive cross-section, higher currents can be transmittedwithout causing heat damage to the material of the contacting element.The large surface area of the contacting material is also believed to beadvantageous because it provides good heat conductivity. It is believedthat the elastic spring component ensures that thermal displacements ofsurrounding components due to different thermal expansion coefficientscan be compensated.

The contacting element may be made of graphite or a conductive ceramicpowder, since these materials are corrosion-resistant. Also, only apredominant portion of the material may be made of graphite orconductive ceramic or metal powder, thus saving on expensive materialswhile achieving approximately the same characteristics. If thesheathed-element glow plug having a contacting element according to theexemplary embodiment of the present invention is manufactured asdescribed below, it is believed that this provides an arrangement of thecomponents located in the glow element housing that preventsshort-circuits. In addition, it is believed to be ensured that thecomponents are pressed together so that they do not come loose or burstdue to the excessive reactive force of elastic elements (e.g., of thecontacting element).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal section through the exemplarysheathed-element glow plug according to the present invention.

FIG. 2 shows a side view of the front section of the external ceramicheater.

FIG. 3 shows the interconnection between the exemplary sheathed-elementglow plug according to the present invention with the controllers.

FIG. 4 shows the exemplary sheathed-element glow plug according to thepresent invention and the resistances in the supply conductors.

FIG. 5 shows a longitudinal section of the exemplary sheathed-elementglow plug according to the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically shows a longitudinal section through an exemplaryceramic sheathed-element glow plug 1 according to the present invention.At its end farther removed from the combustion chamber, sheathed-elementglow plug 1 is electrically contacted through a round plug 2, which isisolated from glow element housing 4 by a gasket 3 and is connected tocylindrical supply conductor 5. Cylindrical supply conductor 5 issecured in glow element housing 4 via a metal ring 7 and an electricallyinsulating ceramic sleeve 8. Cylindrical supply conductor 5 (via acontact pin 10) and a suitable contacting element 12, which may be acontact spring or an electrically conductive powder packing or anelectrically conductive pellet with an elastic spring component that maybe made of graphite, are connected to ceramic glow element 14.Cylindrical supply conductor 5 may also be combined with contact pin 10in a single component. The inside of the sheathed-element glow plug issealed against the combustion chamber by a sealing gasket 15. Sealinggasket 15 is made of an electrically conductive carbon compound. Sealinggasket 15 may, however, also be made of metal, a mixture of carbon andmetal, or a mixture of ceramic and metal. Glow element 14 is made of aceramic heating layer 18 and ceramic supply layers 20 and 21, the twosupply layers 20, 21 being connected by heating layer 18 and, togethervia heating layer 18, forming the conductive layer. Supply layers 20, 21and heating layer 18 may have any desired shape. The conductive layermay have a U shape. Supply layers 20, 21 are isolated via an insulationlayer 22, which is also made of a ceramic material. In the exemplaryembodiment of FIG. 1, glow element 14 is designed so that supply layers20, 21 and heating layer 18 are arranged outside on glow element 14. Atleast supply layers 20, 21 may also be arranged so that they are locatedinside the glow element and are additionally covered by an externalceramic insulating layer. Within the glow element housing, the ceramicglow element is insulated from the other components 4, 8, 12, 15 of theglow element by a glass layer (not illustrated). In order to establishthe electrical contact between contacting element 12 and supply layer20, the glass layer is interrupted at point 24. The glass layer is alsointerrupted for an electrical contact between supply layer 21 and glowelement housing 4 via sealing gasket 15 at point 26. In the exemplaryembodiment, heating layer 18 is located at the tip of the glow elementas a preferred embodiment. This heating layer may also be placed at someother location of the conductive layer. Heating layer 18 should belocated at the point where the greatest heating effect is to beachieved.

FIG. 2 shows a side view of the same ceramic heating element. As in FIG.1, the exemplary embodiment in which heating layer 18 is located at thetip of the glow element is illustrated. Furthermore, supply layers 20,21 and insulation layer 22 can be seen. In this side view, the exemplaryembodiment in which the conducting layer composed of supply layers 20and 21 and heating layer 18 has a U-shaped design is shown.

The operating mode in which the glow element is heated to supportcombustion in the combustion chamber is known as the active mode. Thisheating takes place when the internal combustion engine is started,during a post-glow phase which should extend over 3 minutes, as well asduring an intermediate glow phase when the temperature of the combustionchamber drops excessively during the operation of the internalcombustion engine.

The material of the ceramic sheathed-element glow plug according to anexemplary embodiment of the present invention is selected so that theabsolute electrical resistance of heating layer 18 is greater than theabsolute electrical resistance of supply layers 20, 21. (In thefollowing, the term resistance will be understood to denote absoluteelectrical resistance). In order to avoid cross-currents between theinsulation layer, the resistance of the insulation layer is selected sothat it is considerably greater than the resistance of heating layer 18and supply layers 20, 21.

FIG. 3 schematically shows the devices that communicate withsheathed-element glow plug 1. These include engine controller 30, whichcontains a computing unit and a memory unit. The engine-dependentparameters of the sheathed-element glow plug are stored in enginecontroller 30. These may include, for example, theresistance-temperature characteristic maps as a function of the engineload and engine rpm. The memory of the engine controller also containsone or more temperature reference values for correct combustion. Theengine controller may control parameters that influence combustion, forexample, time of injection, start of fuel injection, and end of fuelinjection. Controller 32 regulates a voltage defined by the enginecontroller. This voltage represents the total voltage used for thesheathed-element glow plug. Controller 32 also houses an ammeter usedfor measuring the current flowing through the glow element. In addition,controller 32 contains a memory and a computing unit. Engine controller30 and controller 32 may also be combined in a single device.

FIG. 4 shows the resistances across the sheathed-element glow plug.Resistance 41 having a value R20 is the resistance of ceramic supplylayer 20. Resistance 43 having value R1 includes the resistance of theheating layer. Resistance 45 having a value R21 includes the resistanceof ceramic supply layer 21. Additional resistances include those of theother supply and return leads, which, however, are believed to be smallcompared with resistances R20 and R21 and are therefore not taken intoaccount. They are not shown in FIG. 4. Resistances 41, 43, and 45 areconnected in series. For the discussions with reference to FIG. 4, anycross-currents that may appear are ignored. Thus, the total resistance Ris composed of the sum of resistances R20, R1, and R21. Resistance R1 isthe greatest summand.

An effective voltage, which is regulated by controller 32, is defined byengine controller 30 using the characteristic maps contained therein andthe desired temperature of the glow element. Due to thetemperature-dependence of resistances 41, 43, and 45, a current I flowsthrough the sheathed-element glow plug, i.e., through resistance R,which is measured in controller 32. The temperature-dependence of thetotal resistance R=R20+R1+R21 is believed to be determined mainly by thetemperature-dependence of resistance R1, since this resistance has thehighest value. The temperature-dependence of resistances R20, 21, andR21 is almost constant over the entire operating range of thesheathed-element glow plugs between room temperature and a temperatureof approximately 1400° C. The temperature of the combustion chamber isin the operating range of the sheathed-element glow plugs.

Using a stored characteristic map, measured current intensity I isconverted by controller 32 into a temperature which results mainly fromthe temperature of heating layer 18 due to the considerably higher valueof resistance R1 compared to resistances R20 and R21. This temperatureis returned to engine controller 30, the effective voltage for thesheathed-element glow plug being redefined on the basis of thetemperature determined.

It is also possible to output the temperature of heating layer 18 of theglow element in another manner, for example, on a display. It isfurthermore possible to draw conclusions concerning the quality ofcombustion in each cylinder from the temperature determined, forexample, taking into account one or more reference temperatures storedin engine controller 30. In the case of incorrect combustion,cylinder-specific measures influencing the combustion can be taken bythe controller to restore correct combustion. For example, the time ofinjection, the start of fuel injection, or the injection pressure can bevaried.

In another exemplary embodiment, the temperature of the combustionchamber can also be measured in the passive mode of the sheathed-elementglow plug, i.e., after the post-glow phase, when the sheathed-elementglow plug is no longer in active operation. In this case, a lowereffective voltage is defined and, as in active operation, current Iflowing through resistance R is measured and thus the temperature of theheating area is estimated, which then corresponds to the temperature ofthe combustion chamber. As in the active mode, the combustion chambertemperature can be compared to one or more reference values for correctcombustion stored in the engine controller for each cylinder. If thecombustion chamber temperature no longer corresponds to correctcombustion, measures can be taken to ensure correct combustion, asdescribed for the active mode of the sheathed-element glow plug; forexample, the injection time, the start of fuel injection and theinjection pressure may be varied.

The value of resistances R20, R1, and R21 and their dependence on thetemperature are set through the temperature-dependence of specificresistance p on the basis of the equation

R=ρ*1/A

where 1 is the length of the resistor and A is the cross-sectional area.The temperature-dependence is obtained from

ρ(T)=ρ₀(T ₀)*(1+α(T)*(T−T ₀))

where ρ(T) is the specific resistance as a function of temperature T; ρ₀is the specific resistance at room temperature T₀, and α(T) is atemperature coefficient, which is temperature-dependent.

In order to achieve a different temperature-dependence of theresistances of supply conductors R20 and R21 as compared to resistanceR1, the specific resistance of heating layer 18 can be selected so thatρ₀ of the heating layer is greater than ρ₀ of the supply layers. As analternative, temperature coefficient α of heating layer 18 can begreater in the operating range of the sheathed-element glow plug thantemperature coefficient α of supply layers 20, 21. It is also possibleto select both ρ₀ and α to be greater for heating layer 18 than forsupply layers 20, 21 in the operating range of the sheathed-element glowplug.

In an exemplary embodiment, the composition of heating layer 18 and ofsupply layers 20, 21 is selected so that ρ₀ of supply layers 20, 21 isat least 10 times smaller than the ρ₀ of heating layer 18. Temperaturecoefficient α of heating layer 18 and of supply layers 20, 21 isapproximately the same. Thus an accuracy of 20 K is achieved in thetemperature measurement over the entire operating range of thesheathed-element glow plug.

In an exemplary embodiment, the specific resistance of insulation layer22 is at least 10 times greater than the specific resistance of heatinglayer 18 in the entire operating range of the sheathed-element glowplug.

In an exemplary embodiment, the heating layer, the supply layers, andthe insulation layer are made of ceramic composite structures containingat least two of the compounds Al₂O₃, MoSi₂, Si₃N₄, and Y₂O₃. Thesecomposite structures can be obtained by a single-stage or multistagesintering process. The specific resistance of the layers can bedetermined by the MoSi₂ content and/or the particle size of MoSi₂; theMoSi₂ content of supply layers 20, 21 may be higher than the MoSi₂content of heating layer 18, with heating layer 18 in turn having ahigher MoSi₂ content than insulation layer 22.

In another exemplary embodiment, heating layer 18, supply layers 20, 21,and insulation layer 22 are made of a composite precursor ceramic havingdifferent filler contents. The matrix of this material is made ofpolysiloxanes, polysilsequioxanes, polysilanes, or polysilazanes, whichmay be doped with boron or aluminum and can be manufactured bypyrolysis. The filler for the individual layers is formed by at leastone of the compounds Al₂O₃, MoSi₂, and SiC. As in the case of theabove-mentioned composite structure, the MoSi₂ content and/or theparticle size of MoSi₂ can determine the specific resistance of thelayers. The MoSi₂ content of supply layers 20, 21 may be set higher thanthat of heating layer 18, with heating layer 18 in turn having a higherMoSi₂ content than insulation layer 12.

The compositions of the insulation layer, the supply layers and theheating layer are selected in the exemplary embodiments described aboveso that their thermal expansion coefficients and the shrinkages of theindividual supply, heating, and insulation layers occurring during thesintering and pyrolysis operations are the same, so that no cracks areformed in the glow element.

FIG. 5 shows another exemplary embodiment of the present invention inthe form of a schematic longitudinal section through a sheathed-elementglow plug 1 according to the present invention. The same symbols used inthe previous Figures denote the same components, which are not explainedhere again. As in FIG. 1, the sheathed-element glow plug illustrated inFIG. 5 has a round plug 2, which is in electrical contact withcylindrical supply conductor 5. Cylindrical supply conductor 5 iselectrically connected to ceramic glow element 14 via contact pin 10 andcontacting element 12. Cylindrical supply conductor 5, contact pin 10,contacting element 12, and ceramic glow element 14 are arranged insequence, in this order, as shown in FIG. 5, in the direction of thecombustion chamber. In the exemplary embodiment illustrated in FIG. 5,the end of ceramic glow element 14 farther removed from the combustionchamber has a journal 11. Journal 11 forms an extension of glow element14 in the direction of the end farther removed from the combustionchamber through a cylindrical lead-through of ceramic supply layers 20,21 and insulation layer 22, journal 11 having a smaller outer diameterthan shoulder 13, the part of glow element 14 which follows in thedirection of the combustion chamber. Furthermore, it is not necessarythat the combustion chamber end of glow element 14 have a heating layer18. In an exemplary embodiment, both supply layers 20 and 21 may beconnected only on the combustion chamber side of the glow element asthey are via heating element 18.

Cylindrical supply conductor 5 and contact pin 10 together form theconnecting element, which can also be designed in one piece. A flangewhich, together with journal 11, delimits contacting element 12 in thedirection of the axis of the sheathed-element glow plug is provided onthe end of the connecting element on the combustion chamber side.

Contacting element 12, which is composed of a pellet made of anelectrically conductive powder, may be made of graphite or metal powderor an electrically conductive ceramic powder. In another exemplaryembodiment, the pellet may be made of an electrically conductive powderpredominantly made of graphite or metal powder or of the electricallyconductive ceramic powder. Due to the design of contacting element 12 asan electrically conductive powder, contacting element 12 guaranteeselastic contact capable of carrying high currents without heat damage.The large surface area of the powder ensures good heat conductivity. Forthe same reason, a low contact resistance can also be achieved togetherwith good conductivity. Graphite and ceramic conductive materials arealso corrosion-resistant. It is believed that the elastic springcomponent of the pellet made of an electrically conductive powderensures that the pellet compensates for thermal displacements of thecomponents due to different heat expansion coefficients.

Laterally, the pellet made of an electrically conductive powder isdelimited by a cylindrical adapter sleeve 9, which is present here as astandalone component instead of the ceramic sleeve 8 shown in FIG. 1.Adapter sleeve 9 is provided, like ceramic sleeve 8, as an insulatingcomponent; in an exemplary embodiment, it is made of a ceramic material.During the manufacture of the sheathed-element glow plug, the pelletmade of electrically conductive powder is pressed in securely betweenthe flange of the connecting element on the end face farther removedfrom the combustion chamber, journal 11 of glow element 14 on thecombustion chamber side end face, and adapter sleeve 9. Being pressed inbetween these fixed components, in particular between the stationarystop of adapter sleeve 9 on ceramic sleeve 8, i.e., the limited pressingheight, prevents the surrounding adapter sleeve 9 from cracking due toan excessive internal pressure buildup due to the pressure exerted oncontacting element 12. Axial pre-stressing of the elastic springcomponent by pressing in the pellet made of electrically conductivepowder, can compensate thermal elongations, settling, and vibrationstresses in the event of vibrating forces acting on the sheathed-elementglow plug.

A sheathed-element glow plug according to FIG. 5 having a pellet made ofan electrical conductive powder as contacting element 12 is manufacturedas follows. Initially sealing gasket 15 is moved from the combustionchamber-side tip of ceramic glow element 14 over ceramic glow element 14and introduced as a composite into glow element housing 4 from the endfarther removed from the combustion chamber. Subsequently contactingelement 12, adapter sleeve 9, connecting element 5, 10, ceramic sleeve8, and metal ring 7 are arranged in a holding element and then alsointroduced from the end farther removed from the combustion chamber intoglow element housing 4. Subsequently, the components in the glow elementhousing are pressed together using an axial force exerted on the endfarther removed from the combustion chamber of metal ring 7; inparticular, contacting element 12, composed of a pellet made of anelectrically conductive powder and sealing gasket 15, are pressed. Aforce is exerted on contacting element 12 only until contact pin 10 ofconnecting element 5, 10 is completely pressed into adapter sleeve 9 andthe end face of ceramic sleeve 8 is in contact with the face of adaptersleeve 9. Pressing the pellet made of an electrically conductivematerial also ensures that the elastic spring component of the pellet ispre-stressed. Subsequently metal ring 7 is locked using a force appliedradially from the outside on glow element housing 4. Seal 3 and roundplug 2 are then installed and also locked using a force applied radiallyfrom the outside on glow element housing 4.

What is claimed is:
 1. A method for making a sheathed-element glow plugincluding a ceramic glow element, a contacting element, the contactingelement including a pellet made of an electrically conductive powder,and a connecting element for supplying power and being electricallyconnected to the ceramic glow element via the contacting element, themethod comprising the steps of: introducing a sealing gasket from a sidetip of a combustion chamber of the ceramic glow element over the ceramicglow element and forming a composite; introducing the composite into aglow element housing; arranging components in a holding element, thecomponents including an adapter sleeve, the connecting element, aceramic sleeve, a metal ring and the pellet made of an electricallyconductive powder; introducing the holding element into the glow elementhousing; pressing the components in the glow element housing using anaxial force that is exerted on an end of the metal ring farther removedfrom the combustion chamber; and locking the metal ring using a forceapplied radially to the glow element housing from outside of it.
 2. Themethod of claim 1, further comprising the step of applying an axialpre-stress to an elastic spring component of the pellet by pressing thecomponents in the glow element housing using an axial force.
 3. Themethod of claim 2, wherein the electrically conductive powder ispredominantly made of at least one of a graphite, a metal powder and anelectrically conductive ceramic powder.
 4. The method of claim 2,wherein the electrically conductive powder is predominantly made of agraphite.
 5. The method of claim 2, wherein the electrically conductivepowder is predominantly made of a metal powder.
 6. The method of claim2, wherein the electrically conductive powder is predominantly made ofan electrically conductive ceramic powder.
 7. The method of claim 1,wherein the electrically conductive powder is predominantly made of atleast one of a graphite, a metal powder and an electrically conductiveceramic powder.
 8. The method of claim 1, wherein the electricallyconductive powder is predominantly made of a graphite.
 9. The method ofclaim 1, wherein the electrically conductive powder is predominantlymade of a metal powder.
 10. The method of claim 1, wherein theelectrically conductive powder is predominantly made of an electricallyconductive ceramic powder.